So today I replaced my stock thermostat with the SLP 160 degree thermo. Never had any cooling problems before. Filled back up with Dexcool 50/50 exactly like directions (already pre-diluted). Within 10 minutes of driving, my car is overheating, got high coolant temp alert, then a/c turning off due to excessive engine heat, then engine overheating--idle engine, which I did and eventually made it back to the house. lucky I was only a few minutes away.

The fans didn't come on until the coolant temp was up in the 230's! I let it cool down and it just kept creeping up again, no fans kicking on. The fluid appears to be moving in the radiator, there are no leaks or unusual noises or anything. Why would the fans not be coming on now if I only replaced the thermostat? fault thermostat? I can't even drive it right now. I will likely put the stock one back in tomorrow morning and see what happens, but that shouldn't have anything to do with when the fans kick in, that should be controlled by the engine computer right?

thermostat installed incorrectly or bad thermostat
More likely, you have trapped air in the cooling system and you need to burp the engine (run to temp with heater full blast, let cool (this is when coolant gets sucked out of the reservoir), check coolant level and add coolant to the correct level (cool), repeat process til coolant level remains at the correct level)

^The proper way to do it is run it with the radiator cap off and keep topping it off. Wait until the fans come on 2 or 3 times and wait for all the bubbles to stop, put the cap back on, and top off the reservoir.

^The proper way to do it is run it with the radiator cap off and keep topping it off. Wait until the fans come on 2 or 3 times and wait for all the bubbles to stop, put the cap back on, and top off the reservoir.

Yep, that way too. I didn't wanna recommend that for safety reasons. If a big bubble of air is trapped in the system and a noob is standing over the hole filling it and the systems does a big burp, there is the potential for a scalded face. The reservoir/recovery bottle is vented and is typically is the highest point in the cooling system and where the burping occurs (air rises to the highest point), hence, my recommendation. Either way works.

Only problem with that is it won't bleed air properly because it needs to build pressure before the radiator cap opens to the reservoir to let out coolant/air. Ideally, you'd put a funnel on the radiator opening and fill it above the neck, so it won't violently slash all over.